MEMS mirrors with precision clamping mechanism

ABSTRACT

A microelectromechanical (MEMS) apparatus has a base and a flap with a portion coupled to the base so that the flap may move out of the plane of the base between first and second position. The base may have a cavity with largely vertical sidewalls that contact a portion of the flap when the flap is in the second position Electrodes may be placed on the vertical sidewalls and electrically isolated from the base to provide electrostatic clamping of the flap to the sidewall. The base may be made from a substrate portion of a silicon-on-insulator (SOI) wafer and the flap defined from a device layer of the SOI wafer. The flap may be connected to the base by one or more flexures such as torsional beams. An array of one or more of such structures may be used to form an optical switch.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is based on and claims priority from Provisionalapplication No. 60/250,081 filed Nov. 29, 2000.

FIELD OF THE INVENTION

[0002] This invention relates generally to microelectromechanicalstructures (MEMS). More particularly, it relates to a clamping mechanismfor MEMS apparatus.

BACKGROUND OF THE INVENTION

[0003] MEMS free-space optical switches can be categorized into twomajor branches: the planar matrix (2-dimensional) approach, and thebeam-steering (3-dimensional) approach. The 2D approach typicallyinvolves mirrors that move between on and off position. The angularaccuracy at the on position is extremely critical as it affects thealignment of the mirror and optical loss of the switch.

[0004] Using <110> silicon with anisotropic etchants, one can formtrenches with 90-degree sidewalls. If one bonds this wafer to anotherwafer that has free rotating mirrors, the sidewall can serve as areference stopping plane to fix the up-mirrors in a vertical position.In addition, the sidewall may also serve as an electrode forelectrostatically clamping the mirror in the vertical position.

[0005] One type of optical switch employsmicroelectromechanically-actuated mirrors. FIG. 1 depicts one type ofMEMS actuated switch 100 that is made using 2 substrates. A top chip 101containing a sidewall for receiving a movable mirror 111 is bonded to abottom chip 102 containing a base 103. There are a few complicationsassociated with the two-wafer approach. The attachment process requiresa very high accuracy aligner-bonder. Moreover, the two-chip processplaces certain geometrical constraints that limit the minimum geometryof the trenches and mirrors. Furthermore, the complexity of thefabrication and alignment process can increase cost and reduce yield.

[0006] Therefore, there is a need in the art for a low-cost, high-yieldscalable switch and a process of fabricating same.

SUMMARY OF THE INVENTION

[0007] The disadvantages associated with the prior art may be overcomeby a microelectromechanical (MEMS) apparatus having a base and a flapwith a bottom portion coupled to the base so that the flap may move outof the plane of the base between first and second angular orientations.An array of one or more of such structures may be used to form anoptical switch. The base may have an opening with largely verticalsidewalls containing one or more electrodes. The sidewalls contact aportion of the flap when the flap is in the second angular orientation.The electrodes may be electrically isolated from the base. The flap mayinclude a magnetic material so that the flap moves in response to anexternal magnetic field. A voltage source may be coupled between theflap and the sidewall electrode to apply an electrostatic force betweenthe sidewall electrode and the flap such that the flap assumes theangular orientation of the sidewall. The electrostatic force may besufficient to prevent the flap from changing position in the presence ofan applied magnetic field. The apparatus may further include anelectrode on the base and a voltage source coupled between the electrodein the base and the flap to apply an electrostatic force between theelectrode in the base and the flap. The base may be made from asubstrate portion of a silicon-on-insulator (SOI) wafer and the flapdefined from a device layer of the SOI wafer. The flap may be connectedto the base by one or more flexures such as torsional beams.

[0008] A MEMS apparatus of the type described above may be provided withone or more conductive landing pads on the underside of the flap thatare electrically isolated from the flap. The landing pads may beelectrically coupled to either the sidewall electrode or the base toreduce stiction and arcing. Alternatively, conductive landing pads maybe provided on the sidewall or base that are equipotential with theflap.

BRIEF DESCRIPTION OF THE FIGURES

[0009]FIG. 1 is a cross-sectional schematic diagram of a MEMS mirrorapparatus according to the prior art.

[0010]FIG. 2A is an isometric schematic drawing of a MEMS apparatusaccording to a first embodiment of the present invention.

[0011]FIG. 2B is an isometric schematic drawing of a MEMS apparatusaccording to an alternative version of the first embodiment of thepresent invention.

[0012]FIG. 3A depicts a simplified cross-sectional schematic diagram ofa MEMS apparatus according to another alternative version of the firstembodiment of the present invention.

[0013]FIG. 3B depicts a simplified cross-sectional schematic diagram ofa MEMS apparatus according to another alternative version of the firstembodiment of the present invention.

[0014] FIGS. 4A-4I depict a series of simplified cross-sectionaldiagrams illustrating the formation of a MEMS apparatus according to asecond embodiment of the invention.

[0015]FIG. 5 is a simplified cross-sectional diagram depicting a portionof the apparatus of FIG. 4I.

[0016]FIG. 6 depicts an optical crossbar switch that uses an array ofMEMS mirrors according to a third embodiment of the present invention.

DETAILED DESCRIPTION

[0017] A first embodiment of the invention is shown in FIG. 2A, whichshows an apparatus 200 having a movable flap that can be preciselyclamped by electrodes on either 0 or 90 degrees surfaces. Such astructure allows the flap to be clamped, for example, in either avertical or horizontal position. Such a flap may be used as part of anarray of several MEMS mirrors in a planar matrix switch.

[0018] The apparatus 200 generally comprises a base 206 and a flap 211coupled to the base 206, e.g. by one or more flexures 214, so that theflap 211 is movable out of the plane of the base 206 from a firstangular orientation to a second angular orientation. By way of example,the first position may be substantially horizontal, i.e., substantiallyparallel to a plane of the base, and the second position may besubstantially vertical, i.e., substantially perpendicular to the planeof the base. The flap 211 may include a light-deflecting element 213 sothat the apparatus 200 may operate as a MEMS optical switch. By way ofexample, the light-deflecting element 213 may be a simple planereflecting (or partially reflecting) surface, curved reflecting (orpartially reflecting) surface, prismatic reflector, refractive element,prism, lens, diffractive element, e.g. fresnel lens, a dichroic coatedsurface for wavelength specific and bandpass selectivity, or somecombination of these. The flap 211 and the base 206 may be formed from aportion of a starting material 201 in order to avoid alignment problemsassociated with post-process bonding associated with a two waferapproach. For example, the starting material 201 may be formed from asilicon-on-insulator (SOI) wafer having a device layer 202, an insulatorlayer 204 and a substrate layer as the base 206. The starting material201 may include an opening or cavity 215 having sidewalls 217 that arevertical, i.e., substantially perpendicular to a plane of the base 206.One or more of the sidewalls 217 may contain an electrode 216 that maybe electrically isolated from the base 206. The flap 211, flexures 214,and sidewalls 217 may be positioned so that a bottom portion of the flap211 contacts one of the sidewalls 217 when the flap 211 is in the secondangular orientation such that the flap 211 may assume an orientationsubstantially parallel to that of the sidewall 217. A voltage appliedbetween the electrode and the flap may attract the flap to the sidewallto secure the flap in place. Preferably, the flap 211 is attracted tothe electrode 216 such that such that the flap 211 may assume theangular orientation of the sidewall 217.

[0019] Any conventional means may be used to provide an actuating forceto move the flap 211. For example, the flap 211 may contain amagnetically active element 240 to facilitate movement of the flap byinteraction with an externally applied magnetic field. The magneticallyactive element 240 may be a magnetically active material having, e.g. afixed magnetic moment, i.e., it may be a permanent magnet. Magneticallyactive materials may include Nickel, Nickel-Iron, Iron-Cobalt,Aluminum-Nickel-Cobalt, Neodymium-Iron-Boron, etc., and, may bedeposited in a uniform or stepped pattern.

[0020] The inventors have discovered that a stepped pattern ofmagnetically active material may be deposited to a movable flap such asthe flap 211. It must be stated that a stepped magnetic material may beused with any moveable flap. The stepped pattern may increase the amountof torque applied to the flap when exposed to a magnetic field. Forexample, the thickness and/or profile of the magnetic material may bevaried by sequentially depositing slabs of material of comparablethickness to produce a series of steps. The height of the steps may varyalong a direction perpendicular to an axis of rotation of the flap. Thestepped magnetic material may act as a guide for the magnetic field andthereby enhance the torque or force exerted on the flap by the field.The configuration of the steps may depend on the relative orientation ofthe magnetic field with respect to the flap and the rotation axis of theflap. By way of example, where the rotation axis is disposed along anedge of the flap and the magnetic field is perpendicular to both therotation axis and a plane of the flap, e.g. the horizontal plane, toenhance the torque to rotate the flap upwards out of the horizontalplane the steps may rise away from the axis. In other words, portions ofthe magnetic material that are close to the axis are lower than portionsthat are further away. To enhance the torque for a downward rotation thesteps may rise toward the rotation axis, i.e., portions of the magneticmaterial that are closer to the axis may be lower than portions that arefurther away.

[0021] The flexures 214 may apply a torsional, or restoring force thatreturns the flap 211 to the first position when the actuating force isremoved. However, other restoring forces may be applied to flap 211 toreturn the flap to the first position. Such torque may be exerted onflap 211 by biasing mechanisms that operate via pneumatic, thermal, ormagnetic principals, including coils that interact with an externalmagnetic field, electrostatic elements, such as gap closing electrodes,piezoelectric actuators and thermal actuators. Multiple restoring forcesmay also used together, and the forces may operate along the same oropposing directions.

[0022] In one configuration, shown in FIG. 2B an apparatus 200B mayinclude a flap 211′ having magnetically active element 240′ thatincludes one or more coils 220 instead of, or in addition to, a magneticmaterial. The coils 220 may interact with an externally applied magneticfield H. The magnetic field H may be applied by a magnetic field source225, which may be any suitable source of magnetic field, e.g. anexternal coil or permanent magnet. By way of example, FIG. 2B depicts amagnetic field that is substantially horizontal. Alternatively, themagnetic field H may have any orientation. Electric current appliedthrough the coil 220 interacts with the external magnetic field H in away that causes a flap 211′ to move from one angular position to anotherwith respect to a base 206′. In a particular configuration, the coil 220may interact with a magnetic material deposited in close proximity tothe flap 211′. The magnetic material may be applied to a sidewall 217′of the base 206′. The magnetic material may be applied through suitabletechniques such as sputtering or electroplating. In configurations wheremagnetically active element 240′ includes a coil 220, the polarity ofcurrent that runs through the coil 220 may be reversed to apply anopposite force to the flap 211.

[0023] By way of example, the one or more coils 220 may be fashioned byforming an insulating layer on the flap 211, etching one or moretrenches in the insulating layer, e.g. in a spiral shape, and fillingthe one or more trenches with electrically conductive material such asaluminum or copper.

[0024] In an alternative embodiment, stiction, e.g., between the flap211 and the base 206, may be reduced by applying a pre-bias force to theflap to move the flap at least partially out of contact with anunderlying base. By way of example, the magnetically active element 240may interact with a fixed pre-bias magnetic field. The pre-bias magneticfield may exert a force on the magnetically active material 240 thatproduces a biasing torque on the flap 211. The biasing torque maypartially counteract a mechanical or other torque exerted on the flap211. As a result, when the flap 211 is in the first position, it ismoved slightly out of a position parallel with the plane of the base206. Consequently, the flap 211 does not touch an underlying portion ofthe base 206. Thus, the effects of stiction and squeeze-film damping maybe reduced.

[0025] It must be stated that a pre-bias force may be applied to avariety of movable MEMS devices, including prior-art MEMS mirrors andflaps, to move the device at least partially out of contact with anunderlying base to reduce effects of stiction. Furthermore, it must bestated that the pre-bias force may be generated by several biasingelements, including but not limited to flap torsion springs, currentcarrying coil, gap-closing electrodes, spring loaded element, stressbearing material, piezoelectric element and thermal bimorph actuator.

[0026] The flap 211 may include a light-deflecting portion 213 so thatthe apparatus may be used in a planar matrix switch. Where such anapparatus is used in a planar matrix switch, it is desirable to be ableto clamp the flap 211 at 2 different positions. Between these twopositions, the accuracy of an ON position, e.g. where the flap 211 isvertical, is of particular importance. In a particular embodiment of thepresent invention, the flap 211 moves out-of-plane by magnetic actuationand is clamped in place by electrostatic attraction to an electrode. Ina similar fashion, the mirror may be held in-plane by another set ofelectrodes or by a voltage difference between the flap 211 and the base206.

[0027] The state or position of a flap such as the flap 211 may besensed by one or more sensors including gap closing electrodes,capacitive, inductive, or piezoresistive elements, strain gauges, coils,magnets, optical sensors, and the like.

[0028] The invention is not limited to flaps that move upwards into an“on” position. For example, an alternative embodiment of a MEMSapparatus 300 is depicted in FIG. 3A. The apparatus 300 may incorporateall the main features of the apparatus of FIG. 2. The apparatusgenerally includes a substrate 301 having a device layer 302, insulatorlayer 304 and base 306. A cavity 315 formed through the substrate 301includes a sidewall electrode 316, and a flap 311 movably connected tothe base 306 by a flexure 314. The flap 311 moves, i.e. translates andmoves, downwards into the through-wafer cavity 315, e.g. under theinfluence of a magnetic field H.

[0029] The flap may include a reflecting or other optical element 313.The apparatus 300 may act as a mirror. The flap 311 may further includeone or more electrically conductive landing pads 322 that areelectrically connected to the sidewall electrode 316. The landing pads322 may be electrically isolated from the flap 311 by an insulatingmaterial 323. By maintaining the landing pads 322 substantiallyequipotential to the sidewall electrode 316, stiction and arcing betweenthe landing pads and sidewall electrode may be reduced. Alternatively,as shown in FIG. 3B an apparatus 300′ of the type described above withrespect to FIG. 3A may include one or more conductive sidewall landingpads 324 that are electrically isolated from a sidewall 317′ andelectrically coupled to a flap 311′.

[0030] The disadvantages associated with the prior art may be overcomeby a process for creating an electrically isolated electrode on asidewall of a cavity in a base. The process generally involves etchingone or more trenches in a backside of a base, e.g., by anisotropic etch.The base may be a crystalline material, e.g., crystalline silicon havinga <110> crystal orientation. The trench may be etched such that anorientation of the sidewall is defined by a crystal orientation of thebase material. A layer of insulating material is formed on one or moresidewalls of one or more of the trenches. A conductive layer is formedon the layer of insulating material on one or more sidewalls of one ormore of the trenches. Base material is removed from a portion of thebase bordered by the one or more trenches to form a cavity in the base.The trenches may be defined underneath a flap that overlies the base.The trench etch may stop on an etch-stop layer so that the cavity doesnot form all the way through the base. The conductive layer maycompletely fill up the trench between the insulating materials on thesidewalls to provide the isolated electrode. Conducting material mayalso be deposited on the backside of the base to provide electricalconnections or electrodes on that side.

[0031] FIGS. 4A-4H depict an example of a process for fabricating astructure of the type shown in FIG. 2A, FIG. 2B, FIG. 3A or FIG. 3B. Thestructure is formed from a starting material wafer 400 having a devicelayer 403, a substrate layer 401 and an insulator layer 402 disposedbetween the device and substrate layers as shown in FIG. 4A. The baseand/or device layer may be made of a crystalline material. For example,the starting material is typically a silicon-on-insulator (SOI) waferwith a silicon <110> handle substrate as the substrate layer 401; thedevice layer can be standard silicon of <100> orientation; and theinsulating layer can be an oxide formed, e.g., by oxidation of a surfaceof the substrate layer 401. Alternatively, the starting material may bea silicon-on-nitride wafer or any other suitable type of wafer materialknown to the art. The device layer 403 may be used, for example to forma mirror plate for a MEMS optical switch, or an array of such switches,such as that shown in FIG. 6. In the first step FIG. 4B, two paralleldeep trenches 404 that reach the buried oxide 402 may be formed in thesubstrate layer 401 from the backside of the wafer 400. The widths ofthe trenches 404 may be sufficiently narrow that they can be completelyfilled-in during a later step. The trenches 404 may define the peripheryof a cavity 415 that may be formed to accommodate a flap similar tothose described above with respect to FIGS. 2-3B. It is also possible toform a single continuous trench that defines the periphery of thecavity. The trench or trenches 405 may be etched by an anisotropicetchant like KOH with silicon nitride or silicon oxide mask (not shownfor simplicity). Alternatively, the trenches 405 may be formed in thesubstrate layer 401 prior to bonding the device layer 403 to thesubstrate layer 401.

[0032] In the next step, as depicted in FIG. 4C, an insulator layer 405may be deposited or grown that covers the interior surface of thetrenches and the wafer backside. The insulator layer 405 may preferablybe a conformally deposited layer (e.g. TEOS oxide, thermal oxide orsilicon nitride). Next, as shown in FIG. 4D, a conformal conductivelayer 406 may be deposited over the insulator layer 405 on the interiorsurface of the trenches 404. In a preferred implementation, theconductive layer 406 completely fills the trenches 404. The conductivelayer 406 may be made of any conductive material, including, but notlimited to a layer of metal or doped semiconductor material. By way ofexample, the conductive layer 406 may be polycrystalline silicon(polysilicon), which is a good candidate material for filling thetrenches 404. Alternately, tungsten, titanium nitride, or siliconcarbide may also be used. After this step, the starting material 400 issubstantially free of any large or deep holes and trenches and may becompatible with additional standard wafer fabrication processes.

[0033] Next, as shown in FIG. 4E, the conductive layer 406 may bepatterned and etched to expose the insulator 405 directly under theto-be-formed cavity. Then a protective material 407 may be deposited onboth a front side (i.e. the exposed surface of the device layer 403) anda backside of the starting material 400. By way of example, theprotective material 407 may be a layer of silicon nitride. A layer ofsilicon nitride can serve as an etching mask on the backside 409 of thestarting material 400 (i.e., the exposed surface of the substrate layer401) in a later step. Silicon nitride can also be used as an insulatorfor the front side of MEMS structures. Alternately, silicon dioxide or apolymer may also be used as the protective material 407.

[0034] Next, a hinged flap 410 may be formed from the top device layer403 as shown in FIG. 4F. The flap may optionally contain landing pads408 on its bottom surface to minimize contact area with the base orsidewall and to provide electrical isolation as described above. Theflap 410 may be mechanically connected to the base through a flexure ortorsional beam similar to those depicted and described with respect toFIG. 2A, allowing the flap 410 to move out of the plane of the base. Areflecting surface 410 may be formed on the surface of the flap 410 sothat the resulting device may function as a mirror.

[0035] Electrically isolated conductive landing pads 422 may be formedon an underside of the flap 410. For example vias may be etched throughthe flap into the oxide 402. The vias may then be partially orcompletely filled with insulating material. The vias may then be filledwith conductive material.

[0036] Up to this point the starting material 400 is free of deep holes.As a result, the starting material 400 is compatible with standard waferfabrication processes and is robust and less likely to be damaged duringhandling and processing.

[0037] At this point a cavity containing a clamping electrode andmechanical stop has been defined. In the next step, as shown in FIG. 4G,the backside masking layer (protective layer 407) may be patterned andan opening 411 may be etched through the backside insulator 405 toexpose the surface of the base 401. In the following step, as shown inFIG. 4H, a selective etch process that does not attack the insulatormaterials (405 and 407) may be used remove the bulk silicon and forms acavity 415 having sidewalls 416. During this step, the conductivematerial 406 can be well protected by the insulator materials 405 and407 and stays intact. The conductive material 406 that filled thetrenches 404 forms sidewall electrodes 417 that are electricallyinsulated by the material of layer 407.

[0038] The bulk material from the base 401 may be removed by anysuitable etch process. For example an anisotropic etch, using an etchantsuch as KOH or other anisotropic etchant (e.g., EDP,etc) may be used.Alternatively, an isotropic silicon etch (e.g., a mixture of Nitricacid, Hydrofluoric acid, Acetic acid, and water) may be used.Furthermore, a vapor etch, e.g. using XeF₂ or SF₆ vapor may be used toetch out the bulk silicon material to form the cavity 415. Finally, asacrificial etch may remove selected portions of insulator layers 405and 402 to release the flap 410, and form a completed MEMS device 490 asshown in FIG. 4I. The flap may still be attached to the rest of thedevice layer 403 and to the base 401 by one or more flexures similar tothose shown and described above with respect to FIG. 2A. Preferably, theflexures are sufficiently flexible to permit the mirror plate to moveout of the plane of the base.

[0039]FIG. 4I shows that the sacrificial layer 402 is completely removedfrom beneath the flap 410. However, in an isotropic etch process, thesacrificial layer 402 may be only partially removed from beneath theother portions of the device layer 403 leaving them undercut. The amountof undercut may be controlled, e.g. by the timing of the etch process.Alternatively, an etch stop material may be used to limit the undercut.

[0040]FIG. 5A depicts a cross-sectional schematic diagram of the device500 that has been fabricated according to a process of the type depictedin FIGS. 4A-4I. FIG. 5A shows the advantages of this process. The device500 has a base 501 with a cavity 515. The device 500 includes a flap 510that is free to move with respect to a plane of the base 501. The flap510 may include a reflecting element 520. The device 500 includessidewall electrodes 517A, 517B that may be electrically isolated fromthe base 501. Voltage sources V₁, V₂, V₃, can apply independentpotentials to the base 501 and electrodes 516A, 516B respectively. Thebase 501 may also be regarded as an electrode if it is electricallyconducting. Alternatively, an electrically isolated electrode (notshown) may be used to clamp the flap 510 to the base when it is in asubstantially horizontal position. The voltage sources V₁, V₂, V₃produce electric fields between the flap 510 and the base 501 orsidewall electrode 516A that can clamp or release the flap 510independently. This may be an important feature for MEMS mirroractuation. Moreover, the sidewall electrodes 516A, 516B on oppositesidewalls 517A, 517B of the cavity 515 can also be electrically isolatedfrom each other by patterning the backside conductor 506 to defineseparate leads to each electrode 516A, 516B.

[0041] The device 500 may include one or more conductive landing pads522 that may be used to reduce stiction between the flap 510 and thesidewall 517A or the base 501. Such landing pads 522 may be electricallyisolated from the flap 510, e.g., by an insulating material, andelectrically coupled to a landing surface, e.g., either the base 501 orthe sidewall 517A. The electrical connection may be maintained invarious configurations, including those where the conductive landingpads 522 are substantially the same electric potential as the landingsurface. This reduces the risk of arcing that can damage the landingsurface or microweld the flap to the landing surface. The base 501 maytherefore serve as an independent electrode for clamping the flap 510 ina position parallel to the plane of the base 501.

[0042] Alternatively, as shown in FIG. 5B, a device 500′, of the typedescribed with respect to FIG. 5A, may achieve the same result byincluding one or more landing pads 524 on a base 501′ and/or sidewall517A′ that are electrically isolated from the base 501′ and a sidewallelectrode 516A′ yet may be electrically connected to flap 510′. The base501′ may therefore serve as an independent electrode for clamping theflap 510′ in a position parallel to the plane of the base 501′.

[0043] The present invention includes systems that incorporate two ormore MEMS apparatus, e.g. arranged in an array. Such an array isdepicted in the crossbar switch 600 of FIG. 6. The switch 600 generallycomprises an array of MEMS mirrors 602 of having features in common withMEMS devices of one or more of the types depicted in FIGS. 2A-2B, FIG. 3or FIG. 5. Specifically, each mirror 602 includes a flap having aportion coupled to a base 601 so that the flap is movable out of theplane of the base 601 from a first angular orientation to a secondangular orientation. Each flap may contain a light deflective element.The base 601 has one or more openings to receive one or more flaps. Eachopening has largely vertical sidewalls. The sidewalls contact a portionof the flap such that the flap assumes an orientation substantiallyparallel to that of the sidewall when the flap is in the second angularorientation. The sidewalls may contain one or more electrodes forclamping the mirrors 602 against the sidewalls. The mirrors 602 coupleoptical signals 604 between one or more input fibers 606 and one or moreoutput fibers 608. Although the mirrors 602 depicted in the apparatus ofFIG. 6 tilt up to deflect the light, those skilled the art willrecognize that the mirrors may alternatively tilt downwards in a mannersimilar to that depicted in FIG. 3.

[0044] In accordance with the foregoing, low-cost, high yield scalableswitches may be provided without the disadvantages attendant to atwo-chip design. It will be clear to one skilled in the art that theabove embodiment may be altered in many ways without departing from thescope of the invention.

1. A microelectromechanical apparatus comprising: a base; a flap havinga portion coupled to the base so that the flap is movable out of theplane of the base from a first angular orientation to a second angularorientation; wherein the base has an opening that receives the flap whenthe flap is in the second angular orientation, the opening having one ormore sidewalls, wherein at least one of the sidewalls contacts a portionof the flap such that the flap assumes an orientation substantiallyparallel to that of the sidewall when the flap is in the second angularorientation; and a sidewall electrode disposed in one or more of thesidewalls.
 2. The microelectromechanical apparatus of claim 1 whereinthe flap further comprises a magnetically active element.
 3. Themicroelectromechanical apparatus of claim 2 wherein the magneticallyactive element is a magnetic material.
 4. The microelectromechanicalapparatus of claim 2 wherein the magnetically active element is a coil.5. The microelectromechanical apparatus of claim 2 further comprising anexternal magnet.
 6. The apparatus of claim 1 wherein the flap isconnected to the base by one or more flexures.
 7. The apparatus of claim7 wherein at least one flexure is electrically conductive.
 8. Themicroelectromechanical apparatus of claim 1 further comprising alight-deflecting element disposed on the flap.
 9. Themicroelectromechanical apparatus of claim 1, wherein the sidewallelectrode is electrically isolated from the base.
 10. Themicroelectromechanical apparatus of claim 1 further comprising: avoltage source coupled between the flap and the sidewall electrode toapply an electrostatic force between the sidewall electrode and theflap.
 11. The apparatus of claim 10 wherein the flap contains amagnetically active material and the electrostatic force between thesidewall electrode and the flap is sufficient to prevent the flap fromchanging position in the presence of an applied magnetic field.
 12. Theapparatus of claim 1 further comprising: an electrode disposed on thebase; and a voltage source coupled between the electrode in the base andthe flap to apply an electrostatic force between the electrode in thebase and the flap.
 13. The apparatus of claims 1 where the base is madefrom a substrate portion of an SOI (silicon-on-insulator) wafer and theflap is defined from a device layer portion of the SOI wafer.
 14. Theapparatus of claim 1 wherein the one or more flexures include one ormore torsional beams.
 15. The apparatus of claim 1, further comprisingone or more conductive landing pads disposed on an underside of the flapwherein the one or more conductive landing pads are electricallyisolated from the flap.
 16. The apparatus of claim 15, wherein one ormore of the conductive landing pads are electrically coupled to asidewall electrode.
 17. The apparatus of claim 15 wherein one or more ofthe conductive landing pads is electrically coupled to the base.
 18. Theapparatus of claim 1 wherein the sidewall includes a sidewall electrodeand one or more conductive landing pads that are electrically isolatedfrom the sidewall electrode.
 19. The apparatus of claim 18 wherein oneor more of the landing pads are electrically coupled to the flap. 20.The apparatus of claim 18 wherein the sidewall electrode is electricallyisolated from the base.
 21. An array of one or more structures, whereineach structure comprises: a base; a flap having a portion coupled to thebase so that the flap is movable out of the plane of the base from afirst angular orientation to a second angular orientation, the flapcontaining a reflecting element; wherein the base has an opening withlargely vertical sidewalls, at least one of the sidewalls containing anelectrode, wherein the sidewalls contact a portion of the flap such thatthe flap assumes an orientation substantially parallel to that of thesidewall when the flap is in the second angular orientation.
 22. Anarray of claim 21 wherein one or more of the structures includes asidewall electrode disposed in one or more of the sidewalls.
 23. Thearray of claim 21, wherein the sidewall electrode is electricallyisolated from the base.
 24. An array of claim 21 wherein the array formsan optical switch.
 25. An apparatus comprising: a flap that is movablefrom a first angular orientation to a second angular orientation; and amagnetic material disposed on the flap, the magnetic material having astepped pattern.
 26. A method for reducing stiction in a MEMS devicehaving a flap that is movable with respect to a base, the methodcomprising: applying a fixed force to the flap to move the flap at leastpartially out of contact with an underlying base.